This invention relates to a protective encapsulant applied to an electronic component package, and, more particularly, to an encapsulant comprising a powdered inorganic filler dispersed in a polycyanurate resin binder formed from a cyanate ester-substituted bisphenol derivative compound.
A typical electronic component package comprises an integrated circuit die formed of a semiconductor silicon chip and mounted onto a printed circuit board or other suitable substrate. Fine metal wire leads are bonded to terminals on the die and on the substrate to interconnect the circuits. The component and the delicate leads are protected by a coating, referred to as an encapsulant or "glob top", which is formed of a powdered inorganic filler dispersed in a polymer resin binder.
Alternately, a component may be mounted onto a substrate by an array of solder bump interconnections that bond terminal pads on the surface of the component to corresponding terminal pads on a facing region of the substrate to attach the component and to connect the circuits. In this package, the component is spaced apart from the substrate by the interconnections. The gap between the component and the substrate and about the interconnections is filled by an encapsulant to reinforce the attachment and to protect the interconnections from corrosion.
Conventional encapsulant compositions have a high content of inorganic powder, typically greater than about 60 weight percent, to provide a coefficient of thermal expansion and other properties compatible with the electronic package materials. For application, the powder is dispersed in a curable liquid vehicle that includes the precursor compounds for forming the binder phase. The mixture is applied to the package and heated to polymerize the binder and thereby complete the encapsulant. Thus, for the encapsulant, a vehicle is selected having a suitably low viscosity to produce a flowable mixture, despite the high solids content, that may be readily dispensed onto the package, preferably at ambient temperature. Further, the binder needs to be curable at a temperature sufficiently low to avoid damage to the package. Common binders are based upon epoxy resins cured with acid anhydride. Such epoxy-base resins tend to absorb moisture and also to contain halide impurities. This combination accelerates corrosion of the leads or interconnections, as well as other metal features of the package. It has been proposed to utilize silicone resins to reduce corrosion, but such resins do not provide adequate mechanical reinforcement.
This invention contemplates an encapsulant for an electronic component package that is formed by applying and curing a mixture of inorganic powder dispersed in a liquid vehicle composed mainly of cyanate ester compounds curable to form a polycyanurate resin binder. The vehicle includes at least one bisphenol-derivative multifunctional cyanate ester compound suitable for formulating a low viscosity liquid that may be blended with a high proportion of solid powder to form a mixture that is dispensable at ambient or near-ambient temperature, yet is curable at a temperature suitably low to avoid damage to the package. The product encapsulant exhibits an enchanted combination of corrosion resistance and mechanical properties, particularly in comparison to conventional epoxy-base encapsulant compositions.